Often, you see on dating advice columns or similar people asking for advice about their current relationships. They have a partner who they love a great deal and have a nice relationship with, but for some reason or other there's a point of incompatibility that, whilst small, is insurmountable. I often see this on reddit where a girl is childfree but her boyfriend wants kids, or a guy is fiscally responsible but his boyfriend throws money around on designer shoes, and many other examples.
In these situations, the sage dispensing their valuable advise will invariably say something along the lines of "You need to break up. I know it's going to be hard, I know you don't want to do this, but you can't stay together so it's best to get it over with."
This seems to stem in some way on the monogamous need to find "the one" - any time you're spending with this "dead end" partner is time you could be searching for "the one". In polyamory, you can date partners who "aren't marriage material" whilst still being able to date partners who are.
I can relate to this situation. Mr Wonderful is just such a partner; he's super important to me right now and my world revolves around him and I want to spend every minute I can snuggled into his shoulder, but I know on a deeper level that we're not compatible at this stage in our lives. (Maybe in 5 years that will change; who knows). I know that if I was monogamous, I would be seriously considering ending things with him because it would be the right long-term strategy so I could go find my prince charming.
But I am so, so happy that I'm not monogamous because I'm not ready to say goodbye to Mr Wonderful.
I might never be.
But I'm also not ready to marry him in the foreseeable future. And there's no problem with that.